What's the most effective intensifier (in terms of shadow detail) for films of ISO 400 +, which are underexposed by1-2 stops?

"He took to writing poetry and visiting the elves: and though many shook their heads and touched their foreheads and said 'Poor old Baggins!' and though few believed any of his tales, he remained very happy till the end of his days, and those were extraordinarily long "- JRR Tolkien, ' The Hobbit '.

Try selenium diluted 1 part toner to 3 parts water. The final result will depend on how much information there is on the underexposed negative. There are other methods such as chromium which I believe is still available from Fotospeed, not too far away from where you are. There is a formula for mercury intensifier that is very good in that it is will certainly intensify very thin negatives. The down side is that it will increase the grain size and mercury is very difficult to obtain.